Semiconductor memory devices are generally classified into volatile memory devices and nonvolatile memory devices. The volatile memory devices have a high read/write speed but lose data stored therein when power supply thereto is interrupted. On the other hand, the nonvolatile memory devices retain data stored therein even when power supply thereto is interrupted. Therefore, the nonvolatile memory devices are used to store data that must be retained regardless of power supply. Examples of the nonvolatile memory devices include Mask Read-Only Memory (MROM) devices, Programmable Read-Only Memory (PROM) devices, Erasable Programmable Read-Only Memory (EPROM) devices, and Electrically Erasable Programmable Read-Only Memory (EEPROM) devices.
Recently, a demand for higher capacity semiconductor memory devices is increasing with a sudden change in the mobile trend. Accordingly, the number of bits stored in one memory cell is increasing. For example, Multi-Level Cell (MLC) memory devices storing multi-bits in one memory cell are being universalized. Also, the number of memory cells integrated in the same area is increasing. However, the reliability of stored data decreases with an increase in the integration density and an increase in the number of bits stored in an MLC.